The present invention relates generally to apparatus for generating and detecting electromagnetic fields, and specifically to non-contact, electromagnetic methods and devices for tracking the position and orientation of an object.
Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
For example, U.S. Pat. No. 4,054,881, incorporated herein by reference, describes a tracking system using three coils to generate electromagnetic fields in the vicinity of the object. The fields generated by these three coils are distinguished from one another by open loop multiplexing of time, frequency or phase. The signal currents flowing in three orthogonal sensor coils are used to determine the object""s position, based on an iterative method of computation.
Other electromagnetic tracking systems are described in U.S. Pat. Nos. 3,644,825, 3,868,565, 4,017,858 and 4,849,692, whose disclosures are likewise incorporated herein by reference.
U.S. Pat. No. 5,391,199, to Ben-Haim, which is incorporated herein by reference, describes a system for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
PCT patent publication No. WO96/05768, whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of non-concentric sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. The sensor coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates.
Radiator coils with cores are known in position sensing systems. The cores increase the field output of the coils, but they tend to distort the fields, and therefore reduce the accuracy of position detection. The theory of magnetic fields generated by radiator coils with cores is known in the art, as described, for example, by John David Jackson in Classical Electrodynamics, Second Edition (1975), pages 168-208, which is incorporated herein by reference. In practice, however, it is difficult to derive a theoretical model that will accurately predict the magnetic field generated by a coil with a core.
Ferrite cores are advantageous, because they have both high magnetic permeability (xcexc) and high resistivity (xcfx81). Due to the high resistivity, the cores can be used with a time-varying (AC) magnetic field without inducing eddy currents in the cores, which further distort and complicate the magnetic field. The Polhemus position-sensing system, as described, for example, in U.S. Pat. No. 4,017,858, uses such ferrite cores in its (AC) radiators. Ferrite materials are relatively expensive and fragile, however, making them impractical and uneconomic for use in sizes over about 5 cm in diameter.
Soft iron cores are also effective in increasing magnetic field output of a coil, but they cause serious distortion of AC magnetic fields due to eddy currents generated in the core by the coil. The Ascension position-sensing system, described in U.S. Pat. No. 4,849,692, is based on a DC magnetic field, and can therefore use soft iron cores in its DC radiator coils, since no eddy currents are generated by the DC field.
The accuracy and efficacy of electromagnetic tracking systems, such as those cited above, is generally dependent on precise knowledge of the distribution of the magnetic fields generated by the radiator coils. Although these fields may be calculated theoretically, based on the geometry of the coils, the actual magnetic fields typically differ from the theoretical models. For example, the fields may differ from the models due to small deviations in the manufacture of the coils. In the case of coils having a ferromagnetic core, the geometry and electrical and magnetic properties of the core must also be taken into account. There will typically be greater deviations from the theoretical models due, for example, to nonlinearities, hysteresis and eddy currents in the core, and to imprecise location of the core relative to the coils. These deviations may lead to inaccuracies in determining the position and orientation of the object being tracked. It would, therefore, be desirable to calibrate the radiator coils by precise measurement of the direction and amplitude of the magnetic field in the vicinity of the object to be tracked.
It is thus an object of some aspects of the present invention to provide a method and apparatus for calibrating electromagnetic radiator coils or other types of magnetic field generators.
In some aspects of the present invention, the field equations of an electromagnetic radiator coil are used to derive a parametric, theoretical model of the field, which is compared with calibration measurements of the field to determine accurate values of the parameters.
In one aspect of the present invention, the theoretical model takes into account perturbations of the field due to the effect of a ferromagnetic core in the radiator coil.
In another aspect of the present invention, the radiator coils are used as part of an object tracking system, such as a system for use in determining the position and orientation of a probe inside the body of a subject during a medical or surgical procedure.
In preferred embodiments of the present invention, apparatus for calibrating magnetic field generators comprises at least one sensor coil, fixed to a positioning device in a known geometrical relation. The positioning device, which may be of any suitable type known in the art, is adapted to position the at least one sensor coil in one or more known positions in a vicinity of the field generator being calibrated. The at least one sensor coil generates electrical signals in the presence of a time-varying magnetic field, which signals are analyzed to determine the direction and amplitude of the magnetic field at the positions of the coils.
In some preferred embodiments of the present invention, the at least one sensor coil comprises a plurality of sensor coils, preferably including three non-concentric coils, which are mutually substantially orthogonal, and are fixed in a predetermined mutual spacing. Non-concentric coils are advantageous in that they may more readily be wound in a small volume, preferably 1 mm3 or less, desired for use in accordance with the present invention.
In some of these preferred embodiments, the coils are fixed in a substantially linear arrangement. Preferably the positioning device positions the coils successively in a plurality of positions along an axis defined by the arrangement of the coils. In one such preferred embodiment, the three non-concentric coils are fixed in a probe substantially as described in PCT patent application No. PCT/US95/01103, whose disclosure is incorporated herein by reference.
In other preferred embodiments of the present invention, the coils are fixed to respective faces of a cube. In one such preferred embodiment, six coils are respectively fixed to the six faces of the cube, such that the axis of each of the coils is orthogonal to the respective face to which it is fixed. Preferably, the positioning device positions the cube in a plurality of positions on a grid defined by the arrangement of the coils on the cube.
In preferred embodiments of the present invention, a method for calibrating a magnetic field generator comprises placing at least one sensor coil in one or more known positions and orientations in a vicinity of the field generator, driving the field generator to generate a time-varying magnetic field, and measuring the electrical signals generated by the at least one sensor coil, so as to determine the direction and amplitude of the magnetic field at the one or more known positions. The coil may have an air core or, preferably, a ferromagnetic core.
In some preferred embodiments of the present invention, wherein the field generator is substantially rotationally symmetrical about an axis thereof, the method for calibrating the field generator includes defining a calibration plane having a first axis defined by an axis of rotational symmetry of the field generator and a second axis chosen to be orthogonal to the first axis. Preferably the second axis is in a plane defined by the field generator. The at least one sensor coil is then placed in one or more known positions that are substantially within a quadrant of this plane, defined by the first and second axes, and the directions and amplitudes of the magnetic fields are determined in this quadrant. Due to the substantial symmetry of the field generator, the directions and amplitudes of the magnetic field determined in this quadrant are sufficient to determine the directions and amplitudes of the magnetic field in any other quadrant defined by choosing another second axis orthogonal to the first axis.
In a preferred embodiment of the present invention, the method for calibrating a field generator includes fixing three sensor coils to a positioning device in known, mutually substantially orthogonal orientations and in known positions in a non-concentric, substantially linear arrangement. The positioning device is used to place the coils successively in a plurality of known positions along a first axis defined by the arrangement of the coils. The electrical signal generated by each of the three sensor coils at each of the plurality of positions along this first axis is used to determine the amplitude of the component of the magnetic field projected along the direction of orientation of the respective sensor coil. Three such component amplitudes are thus determined at each of the plurality of positions, so that the magnetic field is completely determined along the first axis. The positioning device is then shifted to one or more additional axes, parallel to and in known displacement relative to the first axis, and the steps described above are repeated so as to determine the magnetic fields along these additional axes.
Alternatively, in another preferred embodiment of the present invention, fixing the three sensor coils comprises fixing a position sensing device including three sensor coils, substantially as described in the above-mentioned PCT patent application No. PCT/US95/01103. Position signals received from the device at each of the plurality of known positions in the vicinity of the field generator are compared with the actual, known position coordinates, so as to generate a calibration function.
In other preferred embodiments of the present invention, the at least one sensor coil is used to make additional measurements in both the calibration plane, as described above, and one or more additional planes, preferably having the same first axis as the calibration plane, but having different, respective second axes. Such additional measurements are useful in calibrating the field generator when the field may deviate from rotational symmetry, due, for example, to asymmetry and/or eccentricity of a ferromagnetic core within the radiator.
In still other preferred embodiments of the present invention, the at least one sensor coil is used to make measurements of the direction and amplitude of the magnetic field at a grid of points in the vicinity of the field generator.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for calibrating a magnetic field generator, including:
fixing one or more magnetic field sensors to a probe in known positions and orientations;
selecting one or more known locations in a vicinity of the magnetic field generator;
driving the magnetic field generator so as to generate a magnetic field;
moving the probe in a predetermined, known orientation to each of the one or more locations;
receiving signals from the one or more sensors at each of the one or more locations;
processing the signals to measure the amplitude and direction of the magnetic field, at the respective positions of the one or more sensors; and
determining calibration factors relating to the amplitude and direction of the magnetic field in the vicinity of the magnetic field generator.
Preferably, fixing one or more magnetic sensors to a probe includes fixing sensor coils to the probe. Two or more sensor coils are preferably fixed to the probe, in orientations such that respective axes of the coils are mutually substantially orthogonal.
Preferably, fixing one or more magnetic sensors to a probe includes fixing three sensors to the probe, such that the positions of the sensors on the probe are substantially collinear.
Alternatively, fixing one or more magnetic sensors to the probe includes fixing sensors to a cube.
Preferably, selecting one or more known locations includes selecting a plurality of locations, and moving the probe includes moving the probe along an axis defined by the positions of the sensors on the probe and passing through two or more of the plurality of locations, preferably in steps of substantially equal length, such that the distance between any two of the sensors is substantially integrally divisible by the length of the steps.
Preferably, for calibrating a magnetic field generator that is substantially rotationally symmetrical, selecting the one or more known locations includes selecting one or more locations in a quadrant defined by the axis of rotational symmetry of the magnetic field generator and by a second axis in a plane defined by the magnetic field generator and normal to the axis of rotational symmetry, and moving the probe includes orienting the probe so that the one or more sensors are positioned in the plane.
Determining calibration factors preferably includes calculating theoretical values of the amplitude and direction of the magnetic field generated by the magnetic field generator at the one or more known locations; comparing the theoretical values to the amplitude and direction of the magnetic field measured at said locations; and computing arithmetic factors corresponding to the difference between the theoretical values and the measured amplitude and direction of the magnetic field at each such location.
Preferably, computing arithmetic factors includes fitting the theoretical values to the measured amplitude and direction of the field.
Preferably, calculating theoretical values includes deriving a theoretical model of the magnetic field in the presence of an air core within the magnetic field generator, and modifying the model to account for the presence of a ferromagnetic core within the magnetic field generator.
Alternatively or additionally, modifying the model includes determining a perturbation of the field due to a the core, preferably by determining a perturbation due to a nonlinearity of the core or, further additionally or alternatively, by determining a perturbation due to eddy currents in the core.
In a preferred embodiment, the method described above further includes fixing a magnetic-field-responsive position-sensing device to an object; placing the object in the vicinity of the magnetic field generator; receiving signals from the position-sensing device; processing the signals so as to calculate the position or orientation of the object; and applying the calibration factors so as to improve the accuracy of calculation of the position or orientation.
Preferably, calibrating the magnetic field includes storing the calibration factors in a memory associated with the radiator coil.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for calibrating a magnetic field generator including:
a plurality of magnetic field sensors, which generate electrical signals in response to magnetic fields applied thereto by the field generator; and
a positioning device, for moving the sensors,
wherein the sensors are fixed to the positioning device in a substantially linear arrangement, and
wherein the positioning device has an axis of motion that is parallel to an axis defined by the substantially linear arrangement of the coils.
Preferably, the magnetic field sensors include sensor coils, which are fixed so that respective axes of the coils are mutually substantially orthogonal.
Preferably, the magnetic field sensors are fixed to the positioning device in a substantially linear arrangement.
Alternatively, the magnetic field sensors include sensors which are fixed to the faces of a cube, which is fixed to the positioning device.
Preferably, the sensor coils generate signals which are received by a computer which compares the signals to a theoretical model, so as to calibrate the magnetic field generator.
There is also provided, in accordance with a preferred embodiment of the present invention, a calibrated magnetic field generator, including:
at least one coil, which is driven to generate the magnetic field; and
an electronic memory circuit, associated with the at least one coil, for storing calibration factors relating to the field generated by the coil.
Preferably, the field generator includes a core inside the at least one coil, most preferably a ferromagnetic core. Preferably, the ferromagnet includes a ferrite, or alternatively, soft iron.
Preferably, the field generator includes an electronic memory circuit, most preferably an EPROM.
Preferably, the calibration factors relate the field generated by the at least one coil to a theoretical model thereof.
Preferably, the calibration factors include look-up tables.